Semiconductor laser diodes typically comprise a body of a semiconductor material having a p-n or p-i-n junction. When the junction is when appropriately biased, there is generated a flow of charge carriers which recombine to generate light. Index guided laser diodes include a waveguide having an index of refraction which is different from that of the portions of the diode on each side of the waveguide. This difference in the indices of refraction serves to confine the charge carriers to the waveguide and also confine the generated light to the waveguide. Reflection means, such as mirrors or the like, are provided at opposite ends of the waveguide to reflect the light back and forth along the waveguide and thereby form a substantially coherent beam of light. At least one end of the waveguide is partially transparent to allow the generated beam of light to be emitted from the diode. Such laser diodes generally having contacts at the top and bottom surfaces of the body so that the current flows from one electrode to the other vertically through the diode and across the junction.
With the development of optical communication systems which use laser diodes as sources of light, it has been found desirable to form the laser diodes as part of an integrated circuit. Such integrated circuits include other electrical components which form the drive circuit and/or control circuit for the laser diodes. However, to form a laser diode as part of an integrated circuit, it is necessary to have both contacts of the laser diode on the same side of the device. Also, it is desirable that the side of the integrated circuit on which both contacts are formed be planar to conform with the rest of the integrated circuit and thereby allow ease of electrically connecting the laser diode to the other components of the integrated circuit.
Attempts to form a laser diode having both contacts on the same side of the device have resulted in complex structures which are relatively difficult to make. For example, laser diodes have been formed on a conductive substrate of the same conductivity type as one side of the laser diode which is directly on the surface of the substrate. A separate body of semiconductor material of the same conductivity type as the substrate is formed on the substrate surface adjacent to but electrically isolated from the laser diode. The separate body serves as the contact to the side of the laser diode which is directly on the substrate surface. This provides both contacts, i.e., the contact to the other side of the laser diode and the separate body, which are on the same side of the substrate. However, this structure has the disadvantage that it requires extra material for the separate body which takes up additional room on the substrate. In addition, it requires additional processing to form the separate body. Also, if the separate body is electrically isolated from the laser diode by an air gap, this disrupts the planarity of the surface of the integrated circuit.
It is desirable to have a laser diode which has both electrical contacts on the same surface, is relatively easy to fabricate, and has a planar surface which facilitates it use as part of an integrated circuit having other components fabricated therein.